This invention concerns a method to prevent the exhaustion of acid copper plating baths and to recover metallic copper from solutions and sludges containing copper in an ionic form.
The method according to the invention is applied to the treatment of acid copper plating baths employed in plants which plate with copper, for instance, wedding rods or other iron or steel materials that have to be plated with copper, and is also applied to the treatment of fluids containing copper in an ionic form, such as the sewage from plants carrying out electrolytic or chemical engraving of copper, plants which wash copper plated objects, etc.
The method according to the invention is applied to individual copper plating baths and to centralised copper plating baths in which the same acid copper plating solution held in a reservoir is circulated through a plurality of copper plating tanks.
One of the methods of the state of the art for the copper plating of metallic objects made of steel, for instance, consists in the immersion of those objects, for instance in the form of rods, in a tank containing an acid copper plating bath consisting of an aqueous solution of sulphuric acid (H.sub.2 SO.sub.4) and copper sulphate (CuSO.sub.4).
As the object passes through the acid bath, copper ions are reduced and are deposited on the object and cover it, while iron ions pass into solution and form ferrous sulphate.
It has been found that, when the quantity of iron in the acid copper plating bath is greater than 60 grs/lt., copper plating defects appear which make the end product unsatisfactory, and therefore it is necessary to replace the spent acid copper plating bath with a new bath.
It has been found in particular that the copper plating defects consist in the formation of united crystals of ferrous sulphate and copper sulphate to form very hard crystals on the surface of the object thus plated with copper.
Where welding rods are being copper plated, the crystals incorporate also the stearates present as impurities in the bath.
Where rods are concerned, the surface crystals create sliding problems and tend to detach fragments of copper from the surface of the metallic rod.
The exhaustion of the acid copper plating bath entails very serious consequences for the plant inasmuch as these spent acid copper plating baths form toxic and damaging wastes and, as such, have to be disposed of or treated on the spot.
The costs of disposal of these acid baths are very high and therefore make burdensome the method of copper plating such objects.
One method employed to recover at least a part of the copper still contained in the spent acid baths consists of precipitation and of immersing in the spent acid bath iron supports on which is deposited the copper, which is then recovered by shaking and scraping the supports and by decantation, but this system often leads to the recovery of copper containing many impurities and therefore of low quality and not usable as such.
This method does not overcome the problem of treatment of spent acid baths since these spent acid baths, even after partial recovery of the copper by the above method, have to be neutralised with lime. This neutralisation produces a great mass of sludge containing soluble copper and therefore requiring treatment as a toxic and damaging waste.
Another method employed in the treatment of spent acid copper plating baths provides for neutralisation of the spent acid copper plating bath with lime followed by a step of rendering the bath inert, generally performed with cement, thereby producing still more sludge, which has to be disposed of at a dump at a high cost.
According to Italian regulations governing the disposal of sludges (Decree No. 915/82 of the President of the Republic and subsequent changes and supplements) this method enables these toxic and damaging sludges to be derated to the status of special non-toxic and non-damaging sludges, which can be disposed of in dumps of type IIB, but this treatment requires great investments, which increase considerably the cost of disposal of these wastes as regards both the treatment in itself and the transport of the sludges from their production plant to the treatment plant and thence to the dump.
Moreover, this treatment in no way makes possible the recovery of the raw material, mainly copper, in such sludges, as instead was foreseen in the above regulations and was hoped for by the plant operators themselves with a view to reducing the costs of the copper plating treatment.
Furthermore, when the concentration of iron in the acid copper plating bath is higher than 60 grs/lt., incrustations form on the bottom and sidewalls of the tanks containing the acid baths.
These incrustations caused by co-deposition of ferrous sulphate and copper sulphate have to be removed by heavy manual labour.